The present invention relates to a process and apparatus for treating wastes of thermoplastic resin foam in order to prepare them for reuse particularly for reuse in an extrusion process. More particularly, the present invention relates to a process for increasing the apparent density of foamed thermoplastic resin waste material.
It is well known that foamed thermoplastic resin waste materials are difficult to handle due to their bulkiness. In order to decrease the bulk of these waste materials and reduce the area required for their storage, processes are utilized to increase the apparent density of the waste material, allowing it to be reused.
According to conventional processes presently used to increase the apparent density of foamed thermoplastic resin, a pulverized waste material is introduced into a screw extruder to transform the material into a product of higher apparent density. The resulting product is then repelletized in the usual manner. The main drawback with this type of process is the difficulty in feeding the waste material to the extruder screw due to the low apparent density of the waste material. Moreover, the extruder flow rate is reduced as the apparent density of the wastes to be treated decreases. In order to overcome these problems, it has been proposed to use a screw extruder having a conical feed orifice along with an extruder housing having a variable diameter. Due to the cost of such apparatus, however, it is not economically attractive. In addition, this apparatus provides no significant increase in flow rate.
Another problem with the conventional processes using a screw extruder reside in their inability to reduce the water content of the waste materials. The presence of water not only causes the extruder screw to slip, but also results in a product which is improperly vented. Moreover, according to the conventional retreatment processes of thermoplastic resin foam wastes, it is difficult to increase the density of expanded beads to a point where they may be reused as expandable beads for reimpregnation with a blowing agent.
Also in conventional processes using a screw extruder to retreat thermoplastic waste materials containing common additives such as fire retardants, the waste materials degraded rapidly while causing degradation of the extruder itself. Therefore, what is needed is a process to increase the apparent density of foam resin wastes thereby increasing the production flow rate of the processed material, while simultaneously allowing for the treatment of waste material having a very low apparent density. These low apparent density materials typically comprise expandable thermoplastic beads of polystyrene. Also needed is a relatively simple apparatus useful for achieving the above objects.